1. Field of the Invention
The present invention relates to optical communication systems, optical transmitting apparatuses, optical receiving apparatuses, optical communication methods, and storage mediums. More particularly, the invention relates to optical multiplexing transmissions.
2. Description of the Related Art
Optical-fiber communication systems, which permit broadband and long-distance transmissions, are widely used for trunk-line communications in telephone-line networks and data transmission circuit networks. Conventionally, telephone lines for voice communications have mainly been used in communication services. However, with the recent expanding proliferation of the Internet, data communications are now on the rapid increase, and the amount of its use is even beginning to exceed telephone traffic.
Probably, in the future, such a tendency is being more accelerated, and the capacities of trunk lines, on which communications intensively concentrate, are becoming increasingly larger. Thus, similarly, in access networks connected to trunk line networks, there is a growing need for high-speed optical communication systems with large data transmission capacities.
Regarding the optical communication systems, there are provided two communication methods for meeting such increasing communication needs.
One of the methods is referred to as a wavelength division multiplex (WDM) system, which permits a large-capacity transmission. More specifically, in the WDM transmission system, a plurality of light signals having different wavelengths is transmitted through a single optical fiber. With this system, broadband low-loss characteristics of the optical fiber are effectively utilized to achieve the large-capacity transmission.
The other method is referred to as an optical soliton transmission system. An optical soliton is one mode of a light signal transmitted through an optical fiber formed of a nonlinear medium. When a light signal having a certain pulse amplitude ranging from a few picoseconds to a few tens picoseconds is transmitted through the optical fiber as a waveguide at an optical intensity greater than an amplitude of a few milliwatts, both the anomalous dispersion characteristics of the optical fiber and self phase modulation characteristics of the light signal having a great optical intensity can be utilized to transmit the light signal without distorting its optical waveform as a high-speed pulse and in a stable manner regardless of collisions between the light signals. A brief description will be given of the optical soliton transmission system below.
First, the anomalous dispersion characteristics of the optical fiber will be discussed. Even in the case of a homogeneous medium, a refractive index varies with a light wavelength, that is, the refractive index depends on the wavelength. As a result, the propagation velocity V of light varies with the wavelength thereof. Meanwhile, a light signal includes many wavelength components, since the wavelength of a light source used is generally not monochromatic and has a certain spectrum.
Therefore, with the optical propagation velocity varying with the wavelength, as the transmission distance becomes longer, the waveform of the light signal is distorted and the pulse width thereof becomes broader. This phenomenon is called dispersion. Particularly, the sum of material dispersion depending on the wavelength of the light signal and structural dispersion is referred to as chromatic dispersion.
In this case, The frequency of light shifts linearly with respect to transitional positions in a pulse. Specifically, at the leading edge of the pulse, since the frequency shifts to a short wavelength region, a group velocity becomes higher. In contrast, at the falling edge of the pulse, since the frequency shifts to a long wavelength region, the group velocity becomes lower. As a result, the pulse width of the light signal broadens.
Next, the self phase modulation of the light signal will be discussed below. First, when a magnetic field is applied to a dielectric material, the refractive index of the dielectric material changes according to the magnetic field. Then, as shown in the following equation (1), changes in the refractive index are set to be proportional to the square of an electric field (the optical Kerr effect).
xcex94nxe2x88x9d|E|2xe2x80x83xe2x80x83(1)
In the above equation, the symbol n represents the refractive index of the dielectric material, and the symbol E represents an electric field. In general, the greater the amplitude of the light signal, the greater the changes xcex94n in the refractive index, and the higher the refractive index n, the lower the propagation velocity V. (V=c/n: the symbol c represents the velocity of light in a vacuum.) Therefore, the optical intensity of the light signal causes changes in the refractive index, and phase modulation thereby occurs. As a consequence, the light frequency shifts. In the anomalous dispersion wavelength region, at the leading edge of the pulse, the group velocity becomes-lower and the frequency shifts to the short wavelength side. In contrast, at the falling edge of the pulse, the group velocity becomes higher and the frequency shifts to the long wavelength side. That is, the pulse width of the light signal is compressed.
In this way, when the pulse-width broadening of the light signal due to the anomalous dispersion of the optical fiber and the compression of the light signal due to the self phase modulation are kept in balance, the light signal can be transmitted over a long distance while maintaining a certain pulse waveform thereof. That is, the optical transmission can be performed without causing any pulse-waveform distortion. Moreover, an optical soliton has characteristics in which when passings or collisions between light signals occur, no fusion between the solitons occurs and the wavepacket thereof is never collapsed. As if nothing had happened, the solitons continue to propagate just the way they are. Furthermore, since the pulse waveforms of the light signals are not deteriorated, no intercode interference occurs between the output light signals.
FIG. 1 is a structural view showing an optical-soliton transmitting apparatus using the above-described optical soliton transmission system. Referring to FIG. 1, a description will be given of an optical-soliton transmitting apparatus 1.
The optical-soliton transmitting apparatus 1 shown in FIG. 1 includes an output unit 2, a drive signal generating unit 3, an optical fiber 4, a light receiving unit 5, an information signal generating unit 6, and the like. The drive signal generating unit 3 modulates the information signal PS sent from a signal source ch1 into a drive signal PS, which drives the output unit 2. The output unit 2 is formed, for example, by a laser diode to convert the drive signal PS sent from the drive signal generating unit 3 into a light signal LP.
In this case, since the drive signal PS and the light signal LP have specified pulse widths and amplitudes in order to perform an optical soliton transmission. That is, the light signal LP output from the output unit 2 has a great optical intensity so as to perform the optical soliton transmission.
The optical fiber 4 formed of a material having anomalous dispersion characteristics has a structure based on the characteristics. The optical fiber 4 transmits the light signal LP generated by the output unit 2. The optical fiber 4 serves in such a manner that the light signal LP is sent to an end thereof and is then sent to the light receiving unit 5 from the other end thereof. In the optical fiber 4, an optical amplifier, which is not shown here, is disposed to prevent the optical soliton transmission from halting when the light signal LP is attenuated in the optical fiber 4.
The light receiving unit 5 is, for example, formed by a photo diode, and converts the light signal LP transmitted through the optical fiber 4 into drive signal PS. The drive signal PS obtained by the conversion is sent to the information signal generating unit 6, which demodulates the sent drive signal PS into the information signal IS to output.
Next, referring to FIG. 1, a description will be given of the operational example of the optical-soliton transmitting apparatus 1.
First, when an information signal IS from a signal source ch1 is sent to the drive signal generating unit 3, the drive signal generating unit 3 converts the information signal IS into a drive signal PS. When the drive signal PS is sent to the output unit 2, the drive signal PS is converted into a light signal LP.
Then, the light signal LP is sent to the optical fiber 4 to be transmitted through the optical fiber 4. In this situation, the light signal generated by the output unit 2 is transmitted through the optical fiber 4 to the light receiving unit 5, without distorting the pulse waveform of the light signal owing to chromatic dispersion (anomalous dispersion) and self phase modulation.
After this, the light receiving unit 5 converts the light signal LP into a drive signal PS to transmit to the information signal generating unit 6. The information signal generating unit 6 demodulates the drive signal PS into the information signal IS to output.
In FIG. 1, the single information signal IS is transmitted from the single signal source ch1. However, in the optical soliton transmission system, due to a demand for a larger capacity transmission, the implementation of information transmission with signal multiplexing is desired. As a result, there is provided a wavelength-multiplexing soliton-transmission system. In this system, an optical soliton transmission is implemented by using light signals having different wavelengths while multiplexing the wavelengths so as to achieve a large-capacity transmission.
However, when the wavelength-multiplexing soliton-transmission system is used, high precision in the use of laser beams output from an output unit, an optical filter, and the like, are required. Thus, there is a problem in that the above system has a limitation to multiplexing efficiency.
Accordingly, it is an object of the present invention to provide an optical communication system, an optical transmitting apparatus, an optical receiving apparatus, an optical communication method, and a storage medium, in which a large-capacity transmission can be performed at high speed with improved multiplexing efficiency so as to solve the problems describe above.
To this end, according to a first aspect of the present invention, there is provided an optical communication system including an optical transmitting apparatus for converting a plurality of information signals into light signals having different amplitudes according to the respective information signals by providing specified optical intensities permitting optical soliton transmissions to output the light signals, and an optical receiving apparatus having a function of identifying the respective information signals by the amplitudes of the respective light signals input via a transmitting medium having anomalous dispersion characteristics and connecting the optical transmitting apparatus and the optical receiving apparatus to demodulate the light signals into the plurality of information signals.
According to a second aspect of the present invention, there is provided an optical communication method for communicating via a transmitting medium having anomalous dispersion characteristics. The optical communication method includes the steps of converting a plurality of information signals into light signals having different amplitudes according to the respective information signals by providing specified optical intensities permitting optical soliton transmissions and outputting the light signals to the transmitting medium.
In each of the first and second aspects of the present invention, when information is transmitted via the transmitting medium, multiplexing transmission of light signals having different amplitudes is implemented in an optical fiber as the transmitting medium. As a result, with the transmission of optical solitons, a long-distance transmission can be achieved without causing any waveform distortions. Moreover, multiplexing of the light signals permits a large-capacity transmission to be implemented.
According to a third aspect of the present invention, there is provided an optical transmitting apparatus for converting information signals into light signals having specified optical intensities permitting optical soliton transmissions to output the light signals. The optical transmitting apparatus includes a drive signal generating unit for generating drive signals converting the plurality of information signals into the light signals having different amplitudes according to the plurality of information signals and an output unit for outputting the light signals based on the driving signals output from the drive signal generating unit.
In the arrangement of the third aspect of the invention, when the plurality of information signals are converted into the light signals to be output, the drive signal generating unit converts the plurality of information signals into drive signals having different amplitudes according to the respective information signals, and the output unit is driven by the drive signals to output the light signals. Accordingly, the output light signals also have different amplitudes according to the respective information signals. As a result, the individual output light signals corresponding to each of the information signals can be identified by the amplitudes.
In addition, according to a fourth aspect of the present invention, there is provided an optical receiving apparatus for demodulating a plurality of light signals transmitted by optical soliton transmissions into information signals. The optical receiving apparatus includes a light receiving unit for receiving the light signals and an information signal generating unit for generating the information signals based on the amplitudes of the light signals received by the light receiving unit.
In this arrangement, when the amplitude-multiplexed light signals transmitted from the transmitting medium are received by the light receiving unit, according to the amplitude of each of the received light signals, the information signal generating unit generates at least one information signal.
Furthermore, according to a fifth aspect of the present invention, there is provided a storage medium used for communicating via a transmitting medium having anomalous dispersion characteristics. The storage medium stores a program including a conversion step of converting a plurality of information signals into light signals having different amplitudes according to the respective information signals by providing specified optical intensities permitting optical soliton transmissions and an output step of outputting the light signals to the transmitting medium.